The semiconductor industry has recently experienced technological advances that have permitted dramatic increases in circuit density and complexity, and equally dramatic decreases in power consumption and package sizes. Present semiconductor technology now permits single-chip microprocessors with many millions of transistors, operating at speeds of hundreds of millions of instructions per second to be packaged in relatively small, air-cooled semiconductor device packages. A by-product of such high-density and high functionality in semiconductor devices has been the demand for increased numbers of external electrical connections to be present on the exterior of the die and on the exterior of the semiconductor packages which receive the die, for connecting the packaged device to external systems, such as a printed circuit board.
As the manufacturing processes for semiconductor devices and integrated circuits increase in difficulty, methods for testing and debugging these devices become increasingly important. Not only is it important to ensure that individual chips are functional, it is also important to ensure that batches of chips perform consistently. In addition, the ability to detect a defective manufacturing process early is helpful for reducing the number of defective devices manufactured.
To increase the number of pad sites available for a die, different chip packaging techniques have been used. One technique is referred to as a dual in-line package (DIP) in which bonding pads are along the periphery of the device. Another technique, called controlled-collapse chip connection or flip chip packaging, uses the bonding pads and metal (solder) bumps. The bonding pads need not be on the periphery of the die and hence are moved to the site nearest the transistors and other circuit devices formed in the die. As a result, the electrical path to the pad is shorter. Electrical connections to the package are made when the die is flipped over the package with corresponding bonding pads. Each bump connects to a corresponding package inner lead. The resulting packages have a lower profile and have lower electrical resistance and a shortened electrical path. The output terminals of the package may be ball-shaped conductive-bump contacts (usually solder or other similar conductive material) and are typically disposed in a rectangular array. These packages are occasionally referred to as “Ball Grid Array” (BGA). Alternatively, the output terminals of the package may be pins, and such a package is commonly known as the pin grid array (PGA) package.
For BGA, PGA and other types of packages, once the die is attached to the package, the backside portion of the die remains exposed. The transistors and other circuitry are generally formed in a very thin epitaxially grown silicon layer on a single crystal silicon wafer of which the die is singulated from. In a structural variation, a layer of insulating silicon dioxide is formed on one surface of a single crystal silicon wafer followed by the thin epitaxially grown silicon layer containing the transistors and other circuitry. This wafer structure is termed “silicon on insulator” (SOI) and the silicon dioxide layer is called the buried oxide layer (BOX). The epitaxial silicon layer is typically about 10 micrometers thick, the buried oxide layer less than about 1 micrometer, and the bulk silicon greater than 500 micrometers. The transistors formed on the SOI structure show decreased drain capacitance, resulting in a faster switch transistor.
The side of the die including the epitaxial layer containing the transistors and the other active circuitry is often referred to as the circuit side of the die or front side of the die. The circuit side of the die is positioned very near the package. The circuit side opposes the backside of the die. Between the backside and the circuit side of the die is single crystalline silicon and, in the case of SOI circuits, also a buried oxide layer. The positioning of the circuit side provides many of the advantages of the flip chip.
Post manufacture analysis of SOI flip chip dies having a buried oxide layer (BOX) typically is destructive for various reasons, such as a need to remove substrate from the flip chip back side in order to access the circuitry. When too much substrate is removed, or when the removal process is not adequately controlled, the circuitry in the die can be damaged. Optical beam probing of circuitry is one analysis method that is used to analyze a semiconductor die through the substrate. However, intrusion of the optical beam into the SOI circuitry can result in disruption of the operation of, or even damage to, circuitry components. Thus, there is an unmet need for methods of analyzing flips chip BOX die circuit operation without excessively disrupting the die.